Acoustic devices generally include a signal device and a sound transducer. The signal device produces an electrical or pressure modulated input signals corresponding to the sound signal and applies it to the sound transducer. The electro-dynamic loudspeaker is an example of electro-acoustic transducer that converts electrical signals into sound.
A thermoacoustic (TA) device converts the temperature modulation on the heater to pressure waves. The thermoacoustic effect is distinct from the mechanism of the conventional loudspeaker, which the pressure waves are created by the mechanical movement of the diaphragm. When signals applied to the TA element, heating is produced in the TA element according to the variations of the signal and/or signal strength. Heat is propagated into surrounding medium. The heating of the medium causes thermal expansion and produces pressure waves in the surrounding medium, resulting in sound wave generation. Such an acoustic effect induced by temperature waves is commonly called “the thermoacoustic effect.”
There are different types of electro-acoustic loudspeakers that can be categorized by their working principles, such as electro-dynamic loudspeakers, electromagnetic loudspeakers, electrostatic loudspeakers and piezoelectric loudspeakers. However, the various types ultimately use mechanical vibration to produce sound waves, in other words they all achieve “electro-mechanical-acoustic” conversion. Among the various types, the electro-dynamic loudspeakers are most widely used.
There have been several attempts to utilize thin nanoscaled films for thermoacoustic sound generation. A thermophone based on the thermoacoustic effect was created by H. D. Arnold and I. B. Crandall (H. D. Arnold and I. B. Crandall, “The thermophone as a precision source of sound,” Phys. Rev. 10, 22-38 (1917)). They used a platinum strip with a thickness of 700 nm as a TA element. However, the thermophone adopting the platinum strip, listened to the open air, sounds extremely weak because the high thermal inertia of the platinum strip.
The following provide examples of these types of TA devices based on CNTs and photo-lithographically patterned nanowire arrays. Wide frequency response range and relatively high sound pressure level was demonstrated using free-standing CNT thin film loudspeakers [L. Xiao et al., “Flexible, stretchable, transparent carbon nanotube thin film loudspeakers,” Nanoletters 8, 4539-4545 (2008); L. Xiao et al., “High frequency response of carbon nanotube thin film speaker in gases,” J. Appl. Phys. 110, 084311 (2011); K. Suzuki et al., “Study of carbon-nanotube web thermoacoustic loudspeakers,” Jpn. J. Appl. Phys. 50, 01BJ10 (2011); M. E. Kozlov et al., “Sound of carbon nanotube assemblies,” J. Appl. Phys. 106, 124311 (2009); A. E. Aliev et al., “Underwater sound generation using carbon nanotube projectors,” Nano Lett. 10, 2374-80 (2010)] and micro-fabricated arrays of nanowires [A. O. Niskanen et al., “Suspended metal wire array as a thermoacoustic sound source,” Appl. Phys. Lett. 95, 163102 (2009); V. Vesterinen et al., “Fundamental efficiency of nanothermophones: modeling and experiments,” Nano Lett. 10, 5020-24 (2010)]. In another work, thin metallic foil deposited on porous silicon pillars has been used for thermoacoustic sound generation [H. Shinoda et al., “Thermally induced ultrasonic emission from porous silicon,” Nature 400, 853 (1999)].
The most part of patented TA devices are open type systems emitting sound directly to the open air:                1. U.S. Patent Application Publ. No. 20050201575, entitled “Thermally excited sound wave generating device,” (N. Koshida et al.).        2. U.S. Pat. No. 8,019,097, entitled “Thermoacoustic device,” (K. L. Jiang et al.).        3. U.S. Pat. No. 8,019,099, entitled “Thermoacoustic device,” (K. L. Jiang et al.).        4. U.S. Pat. No. 8,059,841, entitled “Thermoacoustic device,” (K. L. Jiang et al.).        5. U.S. Pat. No. 8,068,625, entitled “Thermoacoustic device,” (K. L. Jiang et al.).        6. U.S. Pat. No. 8,073,163, entitled “Thermoacoustic device,” (K. L. Jiang et al.).        7. U.S. Pat. No. 8,300,854, entitled “Thermoacoustic device,” (K. L. Jiang et al.).        8. U.S. Patent Application Publ. No. 20110115844, entitled “Thermoacoustic device with flexible fastener and loudspeaker using the same,” (L. Liu et al.).        
Major limitations exist for the above described open type TA devices. These limitations include low applicable temperatures, sensitivity of nanoscale heaters to the environment and low sound generation efficiency in the low frequency region, where the demand for large size and flexible sound projectors is high.
Accordingly, there is a high need to provide an effective TA device working in harsh environment conditions in air and underwater in the low frequency range.